This invention relates to hypodermic syringes and is directed to the problem of controlling the position of a plunger head in a syringe barrel.
In many instances in medical practice, it is desirable to provide a hypodermic syringe with a liquid dosage of predetermined magnitude well in advance of the time of use. For example, a hypodermic syringe may be prefilled so that it may be instantly available in any emergency that may occur in an operating room or elsewhere. As another example, a nurse may provide a patient with several hypodermic syringes with premeasured doses for self-medication by the patient periodically over a period of time. An increasingly common example is the packaging of prefilled dispensable hypodermic syringes by pharmaceutical manufacturers for maximum convenience to medical personnel.
A serious problem in such practices arises from the fact that when a premeasured medication is placed in a conventional hypodermic syringe well in advance of the time of use, the syringe plunger is necessarily retracted to an exposed position extending well beyond the open end of the syringe barrel, and the exposed plunger may be accidentally or inadvertently advanced to some degree prior to the time of use. The consequence is wasteful loss of at least a portion of the medication and, unfortunately, the unintended reduction in the dosage may not be noticed. In a hospital or a nursing home, prefilled syringes may be kept handy in good supply for instant use, and under such circumstances leakage losses occur even with the greatest care by personnel.
In many instances, syringes are prefilled at a central point by a pharmacist and then transported in quantity to a hospital or nursing home, and the additional handling for transportation greatly increases the possibility of the syringes leaking prior to use.
A pharmaceutical manufacturer shipping prefilled syringes to points of use may take special precautions to guard the extended plungers against accidental forces. One expedient is to place each extended hypodermic syringe in an elongated package designed to afford some protection to the extended plunger. At the time of use, one end of the package is torn off for access to the syringe. Such packages are necessarily of inexpensive construction and therefore not strong enough to fully protect the extended syringes.
One solution to the problem employed by some pharmaceutical manufacturers is to place in one package a cartridge containing a predetermined quantity of medication and a separate specialized syringe barrel designed to receive the cartridge. Typically, the package contains a plastic tray with two elongated cavities therein to nest the cartridge and the specialized springe barrel, respectively. When the package is opened at the time of use, protective plastic caps on the cartridge and the barrel, respectively, must be removed and discarded. Then a retracted plunger head in the cartridge is screwed into engagement with the end of an inner tube in the specialized barrel to cause a long fixed axial needle in the inner tube to pierce the plunger head to place the cartridge medication in communication with the needle at the discharge end of the specialized barrel.
This solution to the problem has numerous disadvantages including: the additional cost of the special package; the additional cost of the separate cartridge; the additional cost of the protective caps for the cartridge and the barrel, respectively; and, finally, the additional cost of the inner tube and axial needle in the specialized barrel. Another disadvantage is that the package is bulky not only because it contains a cartridge as well as a specialized barrel, but also because the capacity of the barrel is in the inner tube, not the barrel proper, and therefore the syringe barrel is necessarily of enlarged diameter.
A further disadvantage is that the syringe is not instantly useable in an emergency. Delay is required by the various steps of opening the package, removing the cartridge and barrel, removing the plastic caps from the cartridge and the barrel, respectively, and then screwing the cartridge into the inner tube of the specialized barrel.
With further reference to the background of the invention, another problem arises that does not relate to prefilled syringes but rather relates to syringes which are filled with medication at the time of use. This problem is the tendency of the plunger of a hypodermic syringe to shift position in response to a fluid-pressure differential across the plunger head.
For example, if a hypodermic needle is inserted in a tubing line for intravenous treatment of a patient, and the plunger of the needle is to be advanced periodically for introducing successive predetermined quantities of medication, the fact that the fluid pressure in the tubing line is always above atmospheric pressure results in a pressure differential across the plunger of the hypodermic syringe that tends to cause the plunger to creep outwardly of the syringe barrel. Thus fluid from the intravenous line backs up into the hypodermic syringe to dilute the medication in the syringe, and there is no way of knowing how far the plunger should be advanced at the end of the next time period to introduce a new predetermined quantity of the medication.
Another example is the manual retraction of the plunger of a hypodermic syringe to create a vacuum in the syringe barrel for drawing fluid into the syringe from a vial or from a body cavity. In such an instance the vacuum tends to advance the plunger, and it is desirable to provide some means to eliminate the necessity for the user to hold the plunger in retracted position over a period of time to resist the effect of the vacuum.